High-speed printer-paper feed engine

ABSTRACT

A high-speed incremental web transport system especially suited for a high-speed printer application includes two motors with velocity feedback driven from a common controller in accordance with a computer originated movement request. Incremental position transducers allow precise repetitive spacing and positional stability of the motor shafts. Differential tension to be created during printing periods is obtained by controlling motor current. Synchronization control for skip-type movements prevents a buildup of positional error.

United States Patent Belson et al.

HIGH-SPEED PRINTER-PAPER FEED ENGINE Inventors:

bo, Chelmsford, both of Mass.

Assignee:

Filed:

Appl. No.: 22,235

Honeywell Inc., Minneapolis, Minn.

Mar. 24, 1970 Ross A. Belson, Natick; Gaston A. Palom- Int. Cl.

Field of Search ..3l8/6, 7, 41, 59, 66, 70, 72, 318/82, 314, 318; 317/6 References Cited UNlTED STATES PATENTS 293Q958 6 ws fll fl a-m-isn Feb. 22, 1972 3,118,097 1/1964 Hettler ..3l8/70 Primary Examiner-Lewis H. Myers Assistant Examiner-H. Huberfeld Attorney-Fred Jacob and Ronald T. Reiling ABSTRACT A high-speed incremental web transport system especially suited for a high-speed printer application includes two motors with velocity feedback driven from a common controller in accordance with a computer originated movement request. Incremental position transducers allow precise repetitive spacing and positional stability of the motor shafts. Differential tension to be created during printing periods is obtained by controlling motor current. Synchronization control for skip-type movements prevents a buildup of positional error.

VELOCITY PHASE g ggg COMPARATOR 33 4 I l I 57 53 COMPUTER 14 Claims, 5 Drawing Figures POSITION r21 TRANSDUCER 7 TENSION REFERENCE VOLTAGE 5 69 73 23 1 POSITION TRANSDUCER Fig 2.

POSITION TRANSDUCER POSITION TRANSDUCER SHEET 1 OF 2 PATENTED F E8 2 2 I972 Fig? 1.

INVENTOR. R088 A. BELSON \KZON PALOMBO BY ATTORNEY LINE LEVEL, GENERATOR RAMP GENERATOR} POSITION TRANSDUCER NULL DETECTOR i 33 f 81 REQUEST LOGIC UNIT COMPUTER PATENTEDFEB 22 m2 SHEET 2 .[]F 2 Fig 4.

I NVEN TOR.

w I Y WM M R HIGH-SPEED PRINTER-PAPER FEED ENGINE BACKGROUND OF THE INVENTION This invention relates to web transport systems. More specifically, the invention relates to a high speed paper transport system for use in printing out information from a computer. 1

The vast amount of printed output generated by todays high-speed computers has created a demand for ever faster printer devices. Traditionally, these devices pririg a line and then advance the paper one or more lines and print another line. Ideally, consecutive lines should be identically spaced with characters, all of which are not simultaneously printed, in a straight line. To speed operation, each line must be printed and the paper transported between printed lines as rapidly as possible.

One traditional approach to computer print-outs has been to use a cylindrical rotating drum of type front above the paper, in conjunction with a set of selectively activated hammers behind the paper to strike the paper against a marking ribbon in order to impress the shape of the character on the drum upon the front of the paper. The rotating drums usually have one row for each character with the identical character in each hammer position. The drum is continually rotated at high speed. Selected print hammers are activated to print all like characters in a given line simultaneously. To prevent character smearing and to have different characters aligned, the paper must be stopped and held during the printing operation. Thus, the basic operation of the apparatus is to move the paper to a new line, stop, and retain the paper precisely in place while the type drum is allowed to revolve at least enough so that all desired characters have passed under the print hammers.

The traditional approach to moving the paper from line to line has been a clutch and brake system or a single motor driving an intricate arrangement of gears and belts which link two or four multiple toothed tractors over which the paper is drawn. In order to obtain good print quality, it is necessary to hold the paper precisely in place under tension during the printing operation.

To accelerate the paper between print lines, the tractors which pull the paper under tension must overcome any contact friction between the paper and a guide surface contacting the paper. This puts additional tension upon the paper. Since tension above a certain critical value will tear the paper, the tension needed to hold the paper taut during printing, plus the necessary acceleration tension must remain below this critical tearing value. Therefore, the tension to hold the paper taut during printing may have to be lower than actually desired.

Moreover, the tension can only be adjusted initially and varies with the mechanical inaccuracies introduced by the various system linkages.

The large inertia of the tractors, with the associated gears and belts, in addition to friction on the paper from the guide surface, requires a large-size motor to achieve rapid paper movement.

Prior art printer systems encounter difficulty in moving the paper accurately and achieving and maintaining an accurate paper position.

Also, for normal printing operations. only one direction of paper movement is required. Certain other types of desired output (e.g., graphs) would be greatly facilitated if a printer could run backwards as well as forwards. Mechanical backlash in the gears and belts, and paper holding mechanizations have heretofore permitted only one direction of paper movement.

It is an object of the present invention to, overcome the foregoing problems and disadvantages. It is a further object to provide an improved web transport system.

SUMMARY OF THE INVENTION The present invention provides an improved positioning control system through the use of two separate motor means with velocity feedback to each motor means to control the two web carrying tractors. The two motor means are driven from a common velocity function generator which, in response to a given computer movement request, generates a velocity waveform to accomplish the desired movement.

According to another feature of the invention, each motor means is also coupled to a position transducer which is selectively connected in a feedback configuration for precise'and positive stop position placement of the rotating portion of each motor means.

According to another feature of the invention, means responsive to the torque generated by the motor means when the web is not moving adds a signal to at least one of the motor means to provide tension of any desired quantity.

According to still another feature of the invention, in order to prevent a buildup in positional difference between the two rotating portions because of inaccuracies of the velocity measuring tachometers, a comparison of the position transducer signals is made and used to augment the velocity command to at least one of the motor means. Preferably, each motor means includes a motor and a drive amplifier. Preferably, each rotating portion includes a shaft.

The invention may be embodied as a basic velocity servosystem with three auxiliary control loops activated under predetermined conditions. The basic system includes two amplifiers for driving the two motors, with two paper carrying tractors and a tachometer coupled to each motor. Both amplifiers, in addition to their respective tachometer signal, also receive a common command from a velocity function generatOI.

According to a feature of the invention, the velocity function generator upon receipt of a paper movement request from a computer, provides a velocity waveform which each amplifier and motor combination can closely follow. The polarity of the velocity waveform causes the motors to run in either a forward or reverse direction.

According to another feature of the invention, the position transducers are each of the multiple null type to produce a null at each possible desired stopping location for the motor shaft and tractors that move the paper. Switch means apply the position signals to the motor means when the final position is reached. Preferably, the switch means maintains the application of the position signals until the next desired paper movement request is received by the velocity function generator and acted upon.

According to still another feature of the invention, the torque responsive means measures the current provided by the amplifiers to each motor and a tension control signal is provided to one of the motor-amplifier combinations to keep a constant current difference between the two motors. Since current is proportional to the torque provided by the motor, a given current difference between the two motors means that a constant tension is provided to the paper web carried between the tractors.

These and other features of the invention are pointed out in the claims. The foregoing summary will become clear and additional modifications will suggest themselves from the following detailed description when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation showing two motors coupled to webcarrying tractors with a tachometer and a position transducer also coupled to each motor shaft.

FIG. 2 is a block diagram showing a system embodying features of the invention.

FIG. 3 is a block diagram detailing the velocity function generator in FIG. 2.

FIG. 4 is an electrical schematic of the ramp generator of FIG. 2.

FIG. 5 shows some typical velocity waveforms from the velocity function generator in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, two electric motors, 1 and 3 drive respective shafts 5 and 7 on which are mounted web carrying tractors 9, 11, I3, and 15. These tractors have radially extending projections 17 which engage with holes 18 in a paper web 19. The paper web 19 is usually horizontally perforated into pages 20 of a length sufi'icient to accommodate about ninety print lines.

The first printed line of a page is known as the head-of-form.

Coupled to the motors 1 and 3 are position transducers 21 and 23 of the magnetoresistive type with multiple poles to provide a multiple null cyclical output for each motor revolution. Two tachometers 25 and 27 are connected to the motor shafts 5 and 7 and provide an output indicative of motor, and hence paper-carrying tractor, velocity.

In FIG. 2, a computer 31 which furnishes the information to be printed ori ginates a request for a desired paper movement. The request is communicated in a digital form over lines 33 to velocity function generator 35. The velocity function generator 35 examines the direction and amount of movement requested and generates an appropriate velocity waveform on line 37 for accomplishing this movement in an optimal manner. The velocity waveform moves between predetermined velocity levels along carefully controlled inclined ramps (see FIG. 5). The slope of the ramp is chosen in order that the motors 1 and 3 with their associated amplifiers 39 and 41 respectively will be able to continuously achieve velocities essentially equal to that commanded by the velocity waveform. Line 37 is connected to amplifiers 39 and 41 through respective summing junctions 43 and 45 which serve to sum together various signals such as the velocity feedback signals provided by tachometers 25 and 27 and the velocity waveform on line 37.

The position transducers 21 and 23 are coupled to the motors 1 and 3 respectively and their output signals are connected through switches 47 and 49 to the summing junctions 43 and 45 respectively. The switches 47 and 49 respond to a decrement or line counter in the velocity function generator 35 to remain open during signals which induce normal motion. The switches 47 and 49 simultaneously close in response to signals from the counter in the velocity function generator. Closing of the switches creates a position loop for precisely holding the tractor at a desired final location. The switches 47 and 49 are simultaneously opened at the start of a paper movement during a signal from the function generator 35 that again induces motion. The switches 47 and 48 may for example comprise FET transistors connected with their respective paths of major current flow (i.e., source to drain) between transducer 21 and summing junction 43 and between trans ducer 23 and summing junction 45 respectively. The velocity function generator 35 controls the control electrodes.

Position transducer 21 is also connnected to the velocity function generator 35 by a line 50 and serves to provide an indication of distance moved. The velocity function generator 35 responds by providing appropriate lower levels of commanded velocity as the motors 1 and 3 approach the final desired position. The signal on line 50 is processed by the velocity function generator 35 to provide an internal signal indicative of a null being traversed. This internal signal is used to decrement the internal line counter (see FIG. 3).

To prevent the mechanical inaccuracies of tachometers 25 and 27 from causing a relative position difference between the two motor shafts 5 and 7 during a long movement of paper, such as the skipping of several pages, the essentially sinusoidal cyclical'signals from position transducers 21 and 23 are fed into squaring amplifiers 51 and 52 respectively. The squaring amplifiers 51 and 52 amplify and clip the sinusoidal input so as to produce a square wave output in phase with the sinusoidal input. The outputs of squaring amplifiers 51 and 52 are passed to a phase comparator 53 which provides an output indicative of the phase difference between the leading edges of the squaring amplifier outputs. The output of phase comparator 53 passes to either line 55 or 57 depending upon which squaring amplifier output edge is leading. The signals on lines 55 and 57 are each applied to both summing junctions 43 and 45 with opposite polarities. The squaring amplifiers may be composed of amplifiers followed by clipping circuits. They may then have further amplification of the clipped signal.

If position transducer 21 indicates that motor 1 is physically in advance of the position of motor 3 as indicated by position transducer 23, phase comparator 53 produces an output on line 57 proportional to the indicated phase difference between position transducers 21 and 23. This signal on line 57 provides an additional signal to summing junction 45 to augment the velocity waveform signal on line 37. The signal at summing junction 43 is treated as a negative signal and thus subtracts from the velocity waveform provided by line 37 This causes motor 1 to slow down and motor 3 to speed up such that the outputs of position transducers 21 and 23 are brought into synchronism. The tachometers 25 and 27 are accurate enough such that the positional difference buildup between the two motors 1 and 3 for a short, low-speed paper movement does not require correction.

Two motor-current-measuring resistors 63 and 65 are connected to a summing junction 67 which provides the difference therebetween to an amplifier 69 whose output is compared with a preset manually adjustable tension reference voltage source 71 by a summing junction 73 and the difference therebetween provided to tension control amplifier 75 whose output is connected to summing junction 43 through switch 77. Switch 77 may also be composed of an FET transistor corresponding to switches 47 and 49. Switch 77 closes and opens in coincidence with switch 47 such that tension is only supplied when the two motors 1 and 3 are stopped. Switches 47, 49, and 77 are all controlled by line 79 from the line counter in the velocity function generator 35. Thus, tension is released before paper movement commences. This tension control loop operates to keep a fixed difference in current through the two motors 1 and 3 when they are stopped. This assures constant tension throughout the printing operation.

FIG. 3 is a block diagram of the internal structure of the velocity function generator 35. Computer 31 produces a digital request signal on lines 33 indicative of the number of print lines the paper 19 is to move. These signals are received by request logic unit 81 which interprets the request signal from computer 31 and produces a number representative of the print lines to be traversed. This number is stored in line counter 83 which makes this number available to a level generator 87 over lines 85. A nonzero number in the line counter 83 produces a signal on line 79 to open switches 47, 49, and 77. When the number is zero, the signal on line 79 will close the switches. Level generator 87 is responsive to the digital number on lines 85 to produce discrete voltage levels on line 89 dependent upon the range of the number indicated by lines 85.

For example, in an actual embodiment of the invention,-

when the number in line counter 83 is one or two, the level generator 87 produces a voltage on line 89 sufficient to drive the motor-amplifier combination at a low velocity of about 25-30 inches per second. If the number in line counter 83 is typically between three and seven, then level generator 87 produces a voltage on line 89 sufficient to drive the motor-amplifier combination .at a velocity of about 70 inches per second. If the number in line counter 83 is above seven, then level generator 87 produces a voltage sufficient to cause a velocity of about inches per second.

Thus ramp generator 91 has an input of discrete voltage levels which are dependent upon the number in line counter 83. In response to the discrete levels on line 89, ramp generator 91 produces an output on line 37 in which the abrupt level changes on line 89 are connected by ramps of controlled slope. The signal on line 37 is that actually supplied to summing junctions 43 and 45.

Position transducer 21 is connected by line 50 to a null detector 82. Null detector 82 is responsive to the null crossings of the position transducer output so as to generate a signal capable of decrementing line counter 83. Thus, one cycle of output from position transducer 21 which is equivalent to a distance of one print line being traversed will decrement the number in line counter 83 by one. Thus, line counter 83 maintains the current count of print lines yet to be traversed.

FIG 4 is an electrical schematic diagram of the ramp generator 91. Equal and opposite supply voltages are supplied at terminals 101 and 103. Zener diodes 105 and 107 in conjunction with resistors 109 and 111 serve to establish substantially constant voltages at the points between the resistors 109 and 111 and the diodes 1 l7 and 133 respectively.

Lead 123 connects the collectors of transistors 121 and 131, the series combination of capacitor 135 and resistor 137, and the input of a buffer amplifier 139. The buffer amplifier 139 prevents leakage of charge stored in the capacitor and the amplifier output is the velocity waveform on line 37. With a quiescent signal level on line 115, zener diode 117 acts in conjunction with resistor 119 and transistor 121 to establish a constant current through resistor 119 and into lead 123. Diode 125 provides temperature compensation for the base emitter junction of transistor 121. Similarly, zener diode 127 acts in conjunction with resistor 129 and transistor 131 to require a constant current from lead 123. Diode 133 serves to temperature compensate for the base emitter junction of transistor 131.

The current set by zener diode 117 to flow through transistor 121 into lead 123 is essentially equal to that set by zener diode 127 to flow from lead 123 through transistor 131. Thus in conjunction with the quiescent level on line 115, transistors 121 and 131 cooperate to pass a preset level of current between the positive and negative voltage supplies. Therefore, no current is supplied to or drawn from capacitor 135 and the voltage on lead 123 remains constant.

Line 37 is connected to input terminal 141 of high-gain amplifier 113. When the signal on the amplifier 113 input lines 89 and 141 are substantially equal, output line 115 will be at its quiescent level. Such a condition is indicative of the fact that the voltage on line 37 is equal to the discrete voltage levels provided on line 89 by level generator 87.

When the paper web is stationary, it is indicative that there is a zero voltage level on line 37, capacitor 135, and on line 89. When a movement request is generated by computer 31, level generator 87 changes the zero voltage level on line 89 to a nonzero level. Therefore, high-gain amplifier 113 causes output line 115 to assume a level other than the quiescent level. For example, if the voltage on line 89 goes from zero volts to some positive voltage value, then the signal on line 115 goes negative. This negative voltage is sufficient to cutoff, transistor 131. Zener diodes 105 and 107 establish voltages that allow the maximum voltage swings on line 115 to positively cutoff transistors 121 and 131. However, transistor 121 will not be affected and will continue to act as a current source. Since the current through transistor 12] into lead 123 can no longer fiow through transistor 131 and resistor 129 to the negative source, it must flow into the series combination of capacitor 135 and resistor 137.

The constant current into capacitor 135 creates a linearly increasing voltage on line 123. The negative voltage on line 115 continues to hold transistor 13] cutoff until the linear increase in voltage across capacitor 135 eventually produces a voltage level from buffer amplifier 139 equal to that present on line 89. Then line 115 returns to its quiescent value and the current through transistor 121 again flows through transistor 131 and produces no further change in the voltage on capacitor 135. Thus line 37 has changed from zero volts to a positive voltage value by means of a controlled linear ramp. This controlled ramp can be followed by the amplifier-motor combination without saturation of the transport servo system.

Resistor 137 in series with capacitor 135 acts to put a small voltage step at the start of the ramp on line 37 by virtue of the current through the resistor. There is no corresponding voltage step at the end of a ramp because of the voltage feedback to amplifier terminal 141. This improves operation of the servo system by providing an increased signal at the beginning of a velocity change operation which serves to overcome the natural lag associated with the transport servo system.

Should the voltage level on line 89 have gone from a zero to a negative value, or from a more positive to a less positive value, a correspondingly opposite sequence would occur. The voltage on line 115 would go positive and cut off transistor 12] causing transistor 131 to pull current out of capacitor 135. This constant current drain from capacitor 135 creates a linearly decreasing voltage on line 123. This linear decrease in voltage continues until the voltage on line 37 is equal to the new level established on line 89.

FIG. 5 shows some typical velocity wavefonns produced on line 37 for various desired movement distances. Curve 151 shows the velocity waveform generated for a movement of one print line. Waveform 153 shows the velocity waveform generated for a movement of six print lines. Waveform 155 shows the velocity waveform produced for a movement of 16 print lines.

The following example will serve to illustrate the overall system performance. Assume motors l and 3 are stopped. Therefore, the velocity function generator has a zero level output on line 37. Switches 47 and 49 are closed so that position transducers 21 and 23 are connected to the respective summing junctions 43 and 45. In addition, switch 77 is closed thus providing a tension signal to summing junction 43.

Position transducer 23 is located at a null by virtue function of the feedback connection and position transducer 21 is slightly offset from null because of the tension control signal provided by amplifier 75. This tension signal is such as to maintain a fixed difference, determined by the preset tension reference voltage 71, between the currents in resistors 63 and Now if computer 31 issues a request over lines 33 to velocity function generator 35 for a desired number of print lines to be traversed, the following steps will occur. After receipt of the request by the request logic unit 81, the computer request is interpreted and the number of lines to be traversed is placed in line counter 83. This immediately causes level generator 87 to produce a nonzero level on line 89 and thus ramp generator 81 starts producing a ramp on line 37 Once a nonzero number exists in line counter 83, switches 47, 49, and 77 will open and motors 1 and 3 by virtue of the velocity feedback loop will assume a velocity consistent with the velocity waveform on line 37. Assuming that the computer 31 requested a long skip, for example 16 print lines, then the signals from position transducers 21 and 23 will be processed by the squaring amplifiers 51 and 52 and phase comparator 53 to produce a corrective output on line 55 or 57 if necessary.

As position transducer 21 indicates a print line traversed by virtue of a null crossing detected by null detector 82, line counter 83 will be decremented and eventually cause level generator 87 to produce a lower voltage level on line 89. Ramp generator 91 will then cause line 37 to linearly approach this new commanded voltage level. Motors 1 and 3 continually closely follow the velocity waveform on line 37. For a requested movement of 16 print lines in the forward direction, the motor velocity would substantially have the shape of waveform in FIG. 5.

When thedesired final position has been reached, line 37 will have returned to 0 volts and switches 47, 49, and 77 are again closed. Thus, position transducers 21 and 23 hold motors 1 and 3 precisely in place while a constant tension is applied by the maintenance of a fixed current difference through resistors 63 and 65.

The summing junctions are composed of known types of adding circuits which where necessary include inversion inputs or outputs so that inputs may be added or subtracted as described.

The synchronizing loop, including elements 21, 23, 51, 52, 53, 43, and 45, allows the maintenance of equal velocity for the two tractors despite the inaccuracies of the tachometers and high motor speeds. It does this by comparing the position transducer signals and generating a velocity correction signal for the motor means in response to the compared signals.

To prevent undue burdening the description with matter within the ken of those skilled in the art, a block diagram approach'has been followed with a detailed functional description of each block and specific identification of the circuitry it represents. The individual engineer is free to select elements and components from his own background or from available standard references.

Although the invention has been described with respect to a high-speed printer application, its extension is readily apparent to other web moving systems. in fact, the invention is applicable to any multiple motor system in which corresponding shaft alignments must be maintained.

From the foregoing discussion it will be apparent that numerous modifications, departures substitutions and equivalences may now occur to those skilled in the art, all of which fall within the true scope and spirit of the present invention.

What is claimed is:

1. A web transport system comprising:

a plurality of rotatable means each in contact with the web for advancing the web;

a plurality of corresponding motor means each of which is drivably connected to a respective one of said plurality of rotatable means;

a plurality of velocity-indicating means each responsive to the velocity of a respective one of said plurality of rotatable means;

a plurality of closed loop velocity feedback means each connecting one of said velocity indicating means to a respective one of said motor means for regulating the speed of said respective motor means;

velocity function generator means for providing a velocity waveform to both of said closed loop velocity feedback means; and

control means connected to said velocity function generator means for varying the velocity waveform.

2. The apparatus of claim 1 wherein said control means includes a position-indicating means responsive to a first one of said motor means to provide an output indicative of the position of said rotating portion.

3. An apparatus as in claim 2 further comprising second position-indicating means responsive to a second one of said motor means to provide an output indicative of the position of said rotating portion;

and two position feedback means each connecting one of said position-indicating means to a respective one of said motor means.

4. A transport system as in claim 3 wherein each of said position feedback means includes switch means responsive to said velocity function generator means for selectively connecting and disconnecting said position feedback means during predetermined portions of the velocity waveform.

5. The apparatus of claim 1 additionally comprising tensionmeasuring means for measuring tension applied by the motor means to the web; and

control loop means responsive to said tension measuring means and to said velocity function generator means for providing a signal to one of said motor means during predetermined portions of the velocity waveform.

6. A transport system as in claim 5 wherein said tensionmeasuring means includes current measuring means for producing signals in response to current in said motor means.

7. The apparatus of claim 5 wherein said tension-measuring means comprises current-measuring means responsive to the current in each of said motor means to provide an output indicative of the difference therebetween;

tension control amplifier means in said control loop means responsive to said current-measuring means and a predetermined value of current to provide an output indicative of the difference therebetween;

and tension control switch means in said control loop means responsive to predetermined portions of the velocity waveform for connecting said tension control amplifier means to one of said motor means.

8. The apparatus ofclaim 3 additionally comprising synchronization loop means including position differencemeasuring means responsive to said position-indicating means for providing an indication of the difference therebetween to one of said motor means.

9. A servosystem comprising a pair of motor means having respective shafts;

two velocity-indicating means responsive to the velocities of said respective shafts to provide velocity indications;

two velocity feedback means each connecting one of said velocity-indicating means to a respective one of said motor means for regulating the speed of said respective shafts;

velocity function generator means for responding to a request signal and providing a velocity waveform to both said motor means;

two position-indicating means each responsive to a respective one of said motor means to provide an output indicative of the respective positions of said shafts; two position feedback means each connecting one of said position-indicating means to a respective one of said motor means and including switch means responsive to said velocity function generator means for selectively connecting and disconnecting said position feedback means during predetermined portions of the velocity waveform; control means responsive to one of said position-indicating means and connected to said velocity function generator rn means for varying the velocity waveform; and

synchronization loop means comprising position difference measuring means responsive to said position indicating means for providing an indication of the difference therebetween to one of said motor means.

10. The apparatus of claim 9, additionally comprising tension-measuring means for measuring tension applied by the motor means to a web transported between said shafts; and

control loop means responsive to said tension-measuring means and to said velocity function generator means for providing a signal to one of said motor means during predetermined portions of the velocity waveform.

11. The apparatus of claim 10 wherein said tension-measuring means includes current-measuring means responsive to the current in each of said motor means to provide an output indicative of the difference therebetween;

tension control amplifier means in said control loop means responsive to said current-measuring means and a predetermined valve of current to provide an output indicative of the difference therebetween; and

tension control switch means in said control loop means responsive to predetermined portions of the velocity waveform for connecting said tension control amplified means to one of said motor means.

12. A high-speed computer line printer comprising a velocity function generator responsive to a computer request signal for generating a velocity waveform;

two amplifiers each with an input terminal having said velocity waveform provided thereto;

two motors each driven by a respective one of said amtwo paper-carrying tractors each coupled to one of said motors;

two velocity-indicating means adapted to provide respective tractor velocity indicating signals to said respective amplifier inputs; two position transducers, each coupled to one of said motors, having cyclical outputs with a null crossing for each possible desired stopping position of said tractors;

two switches each responsive to predetermined portions of the velocity waveform to connect said respective position transducer outputs to s51 respective amplifier inputs; generator means includes a ramp generator for producing a and linear change in output in response to discrete changes in tellsion cont")! means pf to f P ll tension input comprising two equal current sources connected in selng measurement to provide a tension signal to one of said fies;

amplifier inputs during said predetermined portions of 5 the velocity waveform for offsetting one of said tractors from a stopped null position.

a capacitor with a first terminal connected to the junction of said current sources and a second terminal of said capacitor connected to a reference potential; and 13. The apparatus of claim 12 and additionally comprising control means responsive to the difference in voltage across synchronization control means responsive to sald cycllcal position transducer outputs to provide a velocity correction 1 z itf and Input voltage to selectively open signal to one of said amplifier inputs. er 0 Sal Curran Sources 14. The apparatus of claim 1 wherein said velocity function 

1. A web transport system comprising: a plurality of rotatable means each in contact with the web for advancing the web; a plurality of corresponding motor means each of which is drivably connected to a respective one of said plurality of rotatable means; a plurality of velocity-indicating means each responsive to the velocity of a respective one of said plurality of rotatable means; a plurality of closed loop velocity feedback means each connecting one of said velocity indicating means to a respective one of said motor means for regulating the speed of said respective motor means; velocity function generator means for providing a velocity waveform to both of said closed loop velocity feedback means; and control means connected to said velocity function generator means for varying the velocity waveform.
 2. The apparatus of claim 1 wherein said control means includes a position-indicating means responsive to a first one of said motor means to provide an output indicative of the position of said rotating portion.
 3. An apparatus as in claim 2 further comprising second position-indicating means responsive to a second one of said motor means to provide an output indicative of the position of said rotating portion; and two position feedback means each connecting one of said position-indicating means to a respective one of said motor means.
 4. A transport system as in claim 3 wherein each of said position feedback means includes switch means responsive to said velocity function generator means for selectively connecting and disconnecting said position feedback means during predetermined portions of the velocity waveform.
 5. The apparatus of claim 1 additionally comprising tension-measuring means for measuring tension applied by the motor means to the web; and control loop means responsive to said tension measuring means and to said velocity function generator means for providing a signal to one of said motor means during predetermined portions of the velocity waveform.
 6. A transport system as in claim 5 wherein said tension-measuring means includes current measuring means for producing signals in response to current in said motor means.
 7. The apparatus of claim 5 wherein said tension-measuring means comprises current-measuring means responsive to the current in each of said motor means to provide an output indicative of the difference therebetween; tension control amplifier means in said control loop means responsive to said current-measuring means and a predetermined value of current to provide an output indicative of the difference therebetween; and tension control switch means in said control loop means responsive to predetermined portions of the velocity waveform for connecting said tension control amplifier means to one of said motor means.
 8. The apparatus of claim 3 additionally comprising synchronization loop means including position difference-measuring means responsive to said position-indicating means for providing an indication of the difference therebetween to one of said motor means.
 9. A servosystem comprising a pair of motor means having respective shafts; two velocity-indicating means responsive to the velocities of said respective shafts to provide velocity indications; two velocity feedback means each connecting one of said velocity-indicating means to a respective one of said motor means for regulating the speed of said respective shafts; velocity function generator means for responding to a request signal and providing a velocity waveform to both said motor means; two position-indicating means each responsive to a respective one of said motor means to provide an output indicative of the respective positions of said shafts; two position feedback means each connecting one of said position-indicating means to a respective one of said motor means and including switch means responsive to said velocity function generator means for selectively connecting and disconnecting said position feedback means during predetermined portions of the velocity waveform; control means responsive to one of said position-indicating means and connected to said velocity function generator m means for varying the velocity waveform; and synchronization loop means comprising position difference measuring means responsive to said position indicating means for providing an indication of the difference therebetween to one of said motor means.
 10. The apparatus of claim 9 additionally comprising tension-measuring means for measuring tension applied by the motor means to a web transported between said shafts; and control loop means responsive to said tension-measuring means and to said velocity function generator means for providing a signal to one of said motor means during predetermined portions of the velocity waveform.
 11. The apparatus of claim 10 wherein said tension-measuring means includes current-measuring means responsive to the current in each of said motor means to provide an output indicative of the difference therebetween; tension control amplifier means in said control loop means responsive to said currEnt-measuring means and a predetermined valve of current to provide an output indicative of the difference therebetween; and tension control switch means in said control loop means responsive to predetermined portions of the velocity waveform for connecting said tension control amplified means to one of said motor means.
 12. A high-speed computer line printer comprising a velocity function generator responsive to a computer request signal for generating a velocity waveform; two amplifiers each with an input terminal having said velocity waveform provided thereto; two motors each driven by a respective one of said amplifiers; two paper-carrying tractors each coupled to one of said motors; two velocity-indicating means adapted to provide respective tractor velocity indicating signals to said respective amplifier inputs; two position transducers, each coupled to one of said motors, having cyclical outputs with a null crossing for each possible desired stopping position of said tractors; two switches each responsive to predetermined portions of the velocity waveform to connect said respective position transducer outputs to said respective amplifier inputs; and tension control means responsive to a paper tension indicating measurement to provide a tension signal to one of said amplifier inputs during said predetermined portions of the velocity waveform for offsetting one of said tractors from a stopped null position.
 13. The apparatus of claim 12 and additionally comprising synchronization control means responsive to said cyclical position transducer outputs to provide a velocity correction signal to one of said amplifier inputs.
 14. The apparatus of claim 1 wherein said velocity function generator means includes a ramp generator for producing a linear change in output in response to discrete changes in input comprising two equal current sources connected in series; a capacitor with a first terminal connected to the junction of said current sources and a second terminal of said capacitor connected to a reference potential; and control means responsive to the difference in voltage across said capacitor and an input voltage to selectively open either of said current sources. 